Patent application U.S. Ser. No. 85,671, filed 8-14-87, now U.S. Pat. No. 4,795,279, which is assigned to the same assignee as this application, shows a ball bearing device which has separators.
Various satellite systems require heat to be transferred across a rotatable joint. Some of these, like the suspension system for an infra red sensor, require operation at temperatures as low as 10 degrees Kelvin (10.degree. K.), others, like waste heat rejection systems, operate at temperatures in the 300.degree.-400.degree. K. range. In all of these cases, the requirements of low torque and low temperature drop across the joint are important.
There are basically five prior art devices now used for transferring heat across a rotating joint. The following are descriptions of each device, along with limitations of each device.
First, a flexible cable device is provided, wherein flexible cables attach at one end to a stationary side and at the other to a rotating side. Heat is conducted along the length of the cables, so that the shorter the cables, the lower the temperature drop across the joint. Longer cables, however, provide lower torque. Thus, a compromise is required. To complicate this, many materials become stiffer at low temperatures, thus increasing the torque. The main disadvantage of flexible cables is the limited rotation due to cable windup.
Second, a radiation device is provided, which has a series of concentric fans, disposed on both rotating and stationary portions of a rotary joint. The fins are designed to fit inside one another so they do not contact when the unit is assembled. Heat conducted down the length of one set of fins can then be radiated to the other. Because there is no contact between the rotating and stationary fin sets and because of operating in a vacuum there is no viscous coupling, and there is no torque component due to the heat transfer device. The disadvantage of this device is that radiation alone provides poor heat transfer at low temperature differentials. It is therefore difficult, even with very large and heavy radiators, to transfer sufficient heat without unacceptably large temperature differentials.
Third, a radiation and convection device is provided, wherein seals are added to the radiation device, described above, and a gas is used to provide convection heat transfer to augment the radiation. This does reduce the temperature differentials, at equivalent heat transfer rates, to acceptable levels; but the seals present three problems. First, their drag torque is unacceptably high. Second, if they leak, the gas will escape and the heat transfer efficiency will be reduced to that of a pure radiator and the seal torque will remain, thus leaving a device that has high torque and poor heat transfer efficiency. Third, seal performance, drag torque and leak rate, each is a function of operating temperature. The low temperature performance is therefore frequently poorer than room temperature performance.
Fourth, a heat pipe device is provided, wherein a higher heat transfer efficiency than obtained with the radiation and convection device can be obtained. This device is much like the radiation and convection device, except that instead of using a pure gas as a convection medium, a fluid is chosen that will evaporate at the temperature of a hot side and condense at the temperature of a cold side. This system utilizes the latent heat of vaporization of the fluid in order to transfer greater quantities of heat from the hot to the cold side. The same seal limitation exists with this system and an additional limitation of operating temperature range is added. Because of the limited temperature differential and the specific boiling points of various fluids, a different operating fluid is required for each desired operating temperature. This constraint requires exact knowledge of the operating characteristics of the system in advance of the actual use.
Fifth, a rotary fluid coupler device is provided. This coupler does not transfer heat to a rotating device, across an interface and then to a final destination. This coupler uses liquid to absorb the heat from a source, and transfers the liquid across the rotating joint and to a heat sink. The main limitations of this device are high drag torques, and complicated seal and passage design to accommodate two fluid paths.